1. Field of the Invention
The present invention relates to a multipass optical probe, and more particularly to a multipass probe for performing optical interferometric measurements of film properties in conjunction with established optical detection and signal analysis methods.
2. Description of the Related Art
The measurement of thin films on semiconductor and other micro-manufactured parts is typically performed using optical interference techniques in which the reflectance or transmission properties are measured using an optical probe.
Then, the acquired spectra is analyzed with a computer program using known film properties and physics to solve for unknown properties such as film thickness, density etc. For ideal films having one or more perfectly flat layers, these measurements are straight-forward and well documented.
However, in practice, interferometric film thickness and composition measurements on production parts are complicated by the presence of pre-existing structures, topography, and films underneath or embedded in the film being measured and the difficulty of physically moving the sample or part under test to the measurement instrument and aligning it to the instrument for measurement.
Further, the process tool environment that is characteristic of semiconductor and related micro-manufacturing processes is an environment where vibration is common, precise optical alignment is either difficult or impossible, and physical space is constrained. Examples of these tools and environments include plasma deposition tools, chemical mechanical polish (CMP) tools, photoresist application tools, and electro-plating cells, etc. Thus, if one wishes to perform these measurements in the process tool environment, then these factors must be considered.
Existing manufacturers cope with these problems by moving the part being measured from the process environment to a vibration-isolated ex-situ tool, precisely aligning the part, positioning the part and examining a small area containing an easy-to-measure target or region. This operation is accomplished only with considerable time and expense in the form of precision tooling. Current ex-situ tooling is typically large and expensive (0.75M$).
On the other hand, current methods of measuring semi-transparent film thickness and composition using optical interference methods are extremely accurate.
However, prior to the invention, there has been no effective structure or operation of 1) measuring thin film properties of thickness and composition with an inexpensive compact, vibration-insensitive, topography-insensitive and alignment-insensitive optical interference probe; 2) retaining the accuracy advantages inherent in optical interference film measurements; and 3) measuring thin film properties in-situ relative to the process environment.
Further, in order to perform accurate film thickness measurements, conventional techniques often require the precise positioning of a single pass optical beam on a small target (e.g., 100xc3x97100 microns) to avoid the topographic complexity inherent in semiconductor structures. This implies the use of expensive and complicated positioning systems and optics. Since the location of the target is sample specific, many software xe2x80x9crecipesxe2x80x9d must be maintained. This is another problem.
In view of the foregoing and other problems, disadvantages, and drawbacks of the conventional methods and structures, an object of the present invention is to measure thin film properties of thickness and composition with an inexpensive compact, vibration-insensitive, topography-insensitive and alignment-insensitive optical interference probe.
Another object of the invention is to retain the accuracy advantages inherent in optical interference film measurements.
A further object of the invention is to measure thin film properties in-situ relative to the process environment.
In a first aspect of the present invention, a multipass optical probe includes a retroreflective element such that light propagating, in a first direction, from the probe to a sample under test and passing through or reflecting from the sample under test, is reflected back in a second direction opposite the first direction passing through the sample under test a total of at least two times.
In a second aspect of the present invention, a multi-pass optical probe for performing optical thin film thickness and composition measurements, includes an optical light source for emitting light to a sample, a detector for detecting the light reflected from the sample, one of a spectral dispersion element and a filtration element constructed to propagate light from the light source to sample surface films under test, and back to the detector such that a given ray of light passes through substantially a same location on the sample at least two times.
With the unique and unobvious features of the present invention, a probe is constructed such that light propagates from the probe through the sample under test (e.g., either in reflection or transmission) and back to the probe, such that any given ray passes through the test sample two or more times along the same path.
As a result, several significant advantages are derived over the conventional probes including increased interference signal contrast (e.g., higher signal-to-noise ratio), increased rejection of topographic imperfections, increased tolerance of sample alignment variations, and compact, inexpensive construction of the probe.
The invention provides a probe in which thin film properties of thickness and composition can be measured with an inexpensive compact, vibration insensitive, topography insensitive and alignment insensitive optical interference probe, the accuracy advantages inherent in optical interference film measurements are retained, and thin film properties are measured in-situ relative to the process environment.
Further, the aforementioned advantages drive an overall reduction of the number and complexity of sample specific recipes necessary to measure production semiconductors. Indeed, the present invention typically requires approximately {fraction (1/20)}th the number of product specific recipes. The maintenance of measurement recipes is a significant cost item associated with conventional methods that the present invention avoids.